Portugal's ruling centre-right coalition is the clear leader of Sunday's general election, early exit polls said, although it may not hold onto its majority in parliament.

Prime Minister Pedro Passos Coelho's "Portugal Ahead" coalition was ahead with between 36.4 to 43 percent of the vote compared to 29.5 to 35 percent for the opposition Socialists, led by the former mayor of Lisbon Antonio Costa, according to three polls commissioned by television stations.

Long trailing in the polls, the governing coalition of Passos Coelho's Social Democrats and the conservative Popular Party had turned the tables on the opposition in recent weeks, but it is still well short of the 50.4 percent it won with four years ago.

Without an overall majority, Passos Coelho faces being outnumbered in parliament if the parties on the left can bury their differences and join forces.

According to the exit polls, the centre-right could end up with between 100 and 118 seats in the 230-seat chamber.

It needs 116 for an absolute majority.

The Socialist Party was on 77 to 90 MPs, according to the early polls, while the Left Bloc, the sister party of Greece's anti-austerity Syriza, had between 8 and 12 percent of votes which could give it up to 23 seats.

The Communists -- who are allied to the Greens -- were on course for between 12 and 20 seats.

Passos Coelho had campaigned on his record of steering the country through one of the worst economic crises in its history, claiming he had put it back on the path of growth.

When he came to power in 2011, Portugal was on the brink of defaulting on its debts, with his Socialist predecessor Jose Socrates forced to seek a 78-billion-euro ($88-billion) bailout from the European Union and the International Monetary Fund.

Costa and his Socialists had vowed to ease the punishing cuts and tax rises demanded in return for the rescue cash and claimed Passos Coelho slashed spending even further than the lenders had asked for.